Flexible linking piece for stabilising the spine

ABSTRACT

The invention relates to a connecting member for maintaining the spacing between at least two anchor members screwed into vertebrae. It comprises two rigid rod-forming parts ( 12, 14 ) made of a first material and each having a fixing, first portion ( 16, 18 ) adapted to be fixed into an anchor member and a fastening, second portion ( 20, 22 ), said rods ( 12, 14 ) being aligned with each other and said fastening portions ( 20, 22 ) facing each other, and a connecting body ( 24 ) made of a second material which is more elastically deformable than said first material and which interconnects said rigid parts ( 12, 14 ) by means of the facing fastening portions ( 20, 22 ) so that said connecting body ( 24 ) is able to deform elastically, whereby the vertebrae, which are held spaced from each other, are movable relative to each other.

FIELD OF THE INVENTION

The present invention relates to a connecting member for maintaining thespacing between at least two anchor members which are interconnected bysaid connecting member.

BACKGROUND OF THE INVENTION

Fields of application of the invention include stabilization andarthrodesis of segments of the vertebral column in degenerativepathologies of the spine.

Systems for stabilizing the vertebral column by bracing at least twoconsecutive vertebrae by means of anchor members fixed into saidvertebrae and connected by rigid connecting rods are well known in theart. Systems of this kind are generally coupled systems such that twoconsecutive vertebrae are interconnected by two substantially parallelrods fixed one on each side of the spinous processes. The anchor membersare screwed into the posterior portions of the vertebrae and passthrough the pedicles and a substantial portion of the vertebral bodiesand therefore provide a fixed and durable connection.

The above stabilizing systems are routinely used to consolidate severalconsecutive vertebrae. Thus the vertebrae are interconnected by rigidrods over a substantial length of the vertebral column. Such assemblieshold the vertebrae correctly relative to each other; however, theyconsiderably stiffen the spine in terms of bending. It has been shownthat a more flexible stabilizing system, which confers greater relativemobility on the vertebrae, is beneficial in some pathologies.

BRIEF SUMMARY OF THE INVENTION

A first object of the present invention is to provide a connectingmember for maintaining the spacing of existing anchor members while atthe same time allowing relative mobility of said anchor members.

To achieve the above object, a connecting member in accordance with theinvention, adapted to maintain the spacing between at least two anchormembers screwed into vertebrae, comprises at least two rigid rod-formingparts made of a first material and each having a fixing, first portionadapted to be fixed into an anchor member and a fastening, secondportion, said rods being aligned with each other and said fasteningportions facing each other, and a connecting body that is made entirelyfrom a second material that is more elastically deformable than saidfirst material and interconnects the facing fastening portions of saidrigid parts so that said connecting body is able to deform elastically,whereby the vertebrae, which are held spaced from each other, aremovable relative to each other.

Thus a feature of the connecting member lies in the way the two rigidparts are fastened together by means of an elastically deformableconnecting body which imparts relative mobility to the rigid parts whenunder stress, with the reaction force to the stress being proportional,within certain limits, to the deformation of the connecting body. As aresult, the connecting member can be bent by stresses in directions thatare not parallel to the axis of the connecting member; it can also bestretched or contracted by opposing forces acting parallel to the axisof the connecting member.

Consequently, the two anchor members, when at rest, are interconnectedby the connecting member with its fixing portions fastened to the anchormembers, and can be moved relative to each other by forces proportionalto the movement.

Said rigid parts are preferably mechanically connected together by asingle connecting body providing the whole of said mechanicalconnection. In this way a single member provides the connection betweenthe rigid parts at the same time as controlling relative movement of therigid parts. Also, in a particular embodiment, said connecting bodyconsists entirely of a single second material to simplify assembly andto impart homogeneous mechanical properties to it.

The connecting member of the invention advantageously has n rigid partswith n−1 connecting bodies disposed between them along the longitudinalaxis of said member, each rigid part situated between two connectingbodies having one fixing, first portion and two fastening, secondportions, there being one fastening, second portion at each end of saidfixing, first portion, and said fastening, second portions beingconnected respectively to said two connecting bodies, and the rigidparts at the two ends of said member advantageously have respectivesingle fastening, second portions connected to the connecting bodies,whereby said connecting member is adapted to interconnect n anchormembers.

Thus, by virtue of this feature, the connecting member maintains thespacing between all the anchor members that it interconnects, each ofwhich can be fixed to a respective vertebra, to align them. Each rigidpart is fixed to an anchor member and, between successive anchormembers, there is a connecting body that interconnects the two fasteningportions. Thus a single connecting member stabilizes several vertebrae,which reduces the time to assemble the stabilizing system as a whole andconsequently the operating time. Also, by virtue of this feature, theconnecting member stabilizes several consecutive vertebrae by connectingthem together, while at the same time making them highly flexible andconferring on them a high degree of relative compressibility in thelongitudinal direction.

In a preferred embodiment of the invention each of said fasteningportions of said rigid parts that said connecting body interconnects hasa fastening wall to which said connecting body is adapted to adhere.Thus no additional fixing member is needed and the adhesive propertiesof the second material to the fastening wall are sufficient to connectthem.

In one particular embodiment of the invention, said fastening wall hasopenings adapted to cooperate with asperities on said connecting body toincrease the surface area of contact between said wall and said body.

Obviously, providing openings in a wall increases the surface area ofthat wall, which increases the contact area between the two materials ifone of the materials can be molded onto the wall of the other material.The increase in contact area increases the connecting forces betweensaid connecting body and said fastening portions. Also, the staticfriction forces of the material of the connecting member on said twomembers are increased in a corresponding manner and these forces areadded to the connecting forces.

Said second material of which said connecting body is made isadvantageously obtained by polymerization. In this way, the connectingbody can easily be hot molded onto the fastening walls if the materialis polymerized beforehand, or it can be formed in situ if the rate ofpolymerization of the monomers constituting said second material issufficiently low to provide the time necessary for completing theassembly.

In a preferred embodiment of the invention said first material of whichsaid rigid parts are made is a titanium alloy. It is therefore easy toform openings in said fastening wall to which said connecting body isable to adhere.

In another preferred embodiment of the invention, the section of saidrigid rod-forming parts is circular, which facilitates the manufactureof the member. Also, if prior art circular section connecting rods areto be replaced by connecting rods of the invention without making itnecessary to replace the anchor members, it is necessary for said rigidparts to have sections identical to the sections of the prior artconnecting rods.

The present invention also provides a vertebral stabilization system forfastening together at least two vertebrae each having a median planesubstantially perpendicular to the axis of the spine of which they forma part and a posterior wall defining a posterior median plane of saidspine, said system comprising at least two anchor members each adaptedto be fixed into the posterior wall of a respective vertebra so that aline which intersects said two anchor members is substantially parallelto said axis of the spine, which system further comprises at least oneconnecting member of the invention whose two rigid parts are adapted tointerconnect said two anchor members so that the axis of said connectingmember is substantially parallel to said axis of the spine, whereby saidvertebrae, which are interconnected via their posterior portions,present relative mobility along said axis of said spine.

BRIEF DESCRIPTION OF THE DRAWINGS

Other features and advantages of the invention will emerge on readingthe following description of particular embodiments of the invention,which is given by way of non-limiting example and with reference to theaccompanying drawings, in which:

FIG. 1 is a diagrammatic perspective view of a connecting member inaccordance with the invention,

FIG. 2 is a diagrammatic view in axial section of the connecting memberin accordance with the invention,

FIG. 3 is a perspective view showing anchor members connected by theconnecting member,

FIG. 4 is a side elevation view of a vertebral column showing twoconsecutive vertebrae into which there are screwed anchor membersinterconnected by a connecting member in accordance with the invention,and

FIG. 5 is a perspective view showing a connecting member having twoconnecting bodies and three rigid parts of the invention.

DETAILED DESCRIPTION OF THE INVENTION

The various portions of a connecting member of the invention aredescribed initially with reference to FIG. 1.

The connecting member 10 has two cylindrical rigid parts 12 and 14. Eachrigid part 12, 14 has a fixing, first portion 16, 18 and a fastening,second portion 20, 22 forming an enlargement. The facing fasteningportions 20 and 22 are connected together by a connecting body 24 sothat the rigid parts 12 and 14 are in axial alignment. The connectingmember 10 is therefore circularly symmetrical about the axis A.

How the two rigid parts 12 and 14 are fastened together is describedbelow with reference to FIG. 2.

The connecting body 24 is a plastics material body obtained bypolymerization. The material of the body is chosen from materials whichare more elastically deformable than the material of said rigid parts12, 14 and, most importantly, whose elastic properties are of the sameorder of magnitude as those of the posterior ligaments that hold thespine together.

Organic silicon compounds constitute polymers whose mechanicalproperties can be determined by the choice of their basic components, inparticular by their degree of substitution, the nature of thesubstituents, and their molecular weight, and whose elastic behaviorpredominates over its plastic behavior. They therefore constitute afamily of materials suitable for interconnecting the two rigid parts 12and 14. Also, these polymers can adhere strongly to materials ofinorganic composition. Thus the connecting body 24 provides good meansfor fastening together the rigid parts 12, 14, which are generally madeof titanium alloy.

Nevertheless, the polymer materials that can be used are not limited toorganic silicon compounds, and any other material having comparableproperties could be suitable.

The material of the connecting body 24 is adapted to adhere to thefastening walls 20′ and 22′ of said fastening second portions 20, 22.However, to increase the adhesion, openings 30, 32 are formed in thefastening walls 20, 22 of the fastening, second portions and are adaptedto cooperate with asperities 26, 28 on the connecting body 24 which areinserted into the openings 30, 32.

This feature increases the contact area between the two materials andthereby increases the connecting force between them in a directionnormal to said surface of contact and creates static friction forceswhich are additional to the adhesion force.

A connection of the above kind is obtained either by injecting thepolymer while hot between the two rigid parts 12 and 14 held facing eachother in a mold, or by cold molding the mixture of monomers between thetwo rigid parts 12 and 14, if the speed of the reaction is sufficientlylow. The asperities 26, 28 are therefore formed in situ, when thepolymer liquid or paste inserted into the openings 26, 28 solidifiesafter cooling or after a chemical reaction. Obviously, the connectingbody 24 consists of the polymer disposed between the rigid parts 12 and14, more specifically between the fastening walls 20′ and 22′, and, inorder to retain the polymer between the facing portions while it is inthe liquid state, the walls of the mold must necessarily surround thespace between and in line with the two rigid parts 12, 14.

In a particular embodiment (not shown) the openings 30, 32 formed in thefastening walls 20′ and 22′ open onto the outside wall of the rigidparts 12 and 14 so that the liquid polymer penetrates entirely into theopenings 30, 32 without it being possible for air to be trapped therein.This reinforces the fastening of the connecting body 24 to the rigidparts 12, 14.

Also, the openings 30, 32, which are shown as parallel to thelongitudinal axis of the connecting member in FIG. 2, can be oblique tothat longitudinal axis and/or not rectilinear. These configurationsincrease the static friction forces of the polymer on the rigid parts,which fastens them together more strongly.

Now that the manner in which the two rigid parts are fastened togetherhas been described, movement of the rigid parts relative to each otheris described with reference to FIG. 1.

Given the circular symmetry of the rigid parts 12 and 14 and theconnecting body 24, and the nature of the material of the connectingbody 24, the connecting member 10 is able to bend in all directions in aplane Pp perpendicular to the axis A of the connecting member when thetwo first portions are immobilized. Bending of the connecting member 10compresses one edge of the connecting body 24 and stretches thediametrally opposite edge, whereas the rigid parts 12 and 14 retaintheir shape. Because the material of the connecting body 24 iselastically deformable, when the stresses causing the bending areremoved, the connecting member 10 returns to its original state in whichthe rigid parts 12 and 14 are in axial alignment.

Also, the rigid parts 12 and 14 can move relative to each other inopposite directions along the longitudinal axis A to compress or stretchthe connecting body 24.

The relative movement of the two rigid parts 12 and 14 can occur indirections other than the directions described above, but the connectingmember is principally loaded in bending, tension and compression, asdescribed in more detail below.

Deformation of the connecting member connected with relative movement ofthe anchor members 42 and 44 is described next with reference to FIG. 3.

FIG. 3 shows the connecting member 10 whose two rigid parts 12 and 14interconnect the two anchor members 42 and 44. The two anchor members 42and 44 are parallel to each other in a common axial plane Pa.

Each anchor member 42, 44 has a threaded shank 46 with a U-shaped head48 at the top whose inside wall is threaded so that a screw-formingmember 50 can be screwed into it. Thus the first portions 16 and 18 ofthe rigid parts 12 and 14 are accommodated in the heads 48 of therespective anchor members 42 and 44 and are locked to them by tighteningthe screw-forming members 50.

As a result, when the threaded shanks 46 of the anchor members movetowards each other due to the effect of opposite forces T and −T in theplane Pa and substantially parallel to the axis A the anchor members 42and 44 deform the connecting member, which bends.

The bending of the connecting member 10 compresses the lower edge 52 ofthe connecting body 24 and stretches the diametrally opposite upper edge54, while the rigid parts 12 and 14 retain their shape. Because thematerial of the connecting body 24 is elastically deformable, when thestress is removed the connecting member reverts to its originalrectilinear shape and the threaded shanks of the anchor members 46return to their former relative position.

The mechanism of elastic bending of the connecting member 10 and theanchor members 42, 44 described above is the same if the threaded shanks46 of the anchor members 42 and 44 move away from each other, theconnecting member bending with the opposite curvature.

Also, the anchor members 42 and 44 are movable in translation relativeto each other along the axis A, their relative movement stretching orcompressing the connecting body 24.

The use of the connecting member 10 in a vertebral stabilization systemfor fastening together at least two vertebrae V1 and V2 is describedbelow with reference to FIG. 4.

The vertebrae V1, V2 each have respective median planes PV1, PV2substantially perpendicular to the axis Ar of the spine of which theyform part, and respective posterior walls PPV1, PPV2 defining aposterior median plane PPr of said spine.

The stabilizing system includes at least two anchor members 42 and 44respectively screwed into the posterior walls PPV1, PPV2 of thevertebrae V1, V2, so that a line L that intersects the two anchormembers 42 and 44 is substantially parallel to said axis Ar of thespine. The two first portions 16 and 18 of the connecting member 10interconnect the two anchor members 42 and 44. As a result, thevertebrae V1 and V2, which are interconnected in their posteriorportions, possess relative mobility along the axis Ar of the spine.

Thus when the spine is stretched, the vertebrae V1 and V2 move away fromeach other in opposite directions E and −E, which causes the threadedshanks 46 to move away from each other, deforming the connecting member10, and in particular its connecting body 24. This is because theconnecting body is compressed both longitudinally and at the upper edge54. The deformed connecting member has it concave side facing away fromthe spine.

When the spine is bent, the inverse effect occurs and the vertebrae V1and V2 move towards each other, which induces deformation of theconnecting member with its concave side facing toward the spine.

The connecting body is then subjected to longitudinal extension of itsupper edge 54 and possibly to compression of its lower edge 52.

It will be understood that the connecting member 10 in accordance withthe invention achieves greater relative mobility of the vertebraecompared to the prior art connecting rods, which cannot be compressedlongitudinally.

In a particular embodiment as shown in FIG. 5, the connecting member hasthree rigid rod-forming parts 12, 14, 15, and two connecting bodies 24₁, 24 ₂ interconnecting the three rigid parts 12, 14, 15. To this end,the central rigid part 15 includes a fixing, first portion and twofastening, second portions, with one fastening, second portion on eachside of said fixing, first portion. The fastening, second portions areconnected to the two connecting bodies 24 ₁, 24 ₂. The other two rigidparts 12, 14, situated at the two ends of the connecting member, have asingle fastening, second portion connected to the connecting bodies.

The connecting member therefore maintains the spacing between threeanchor members that it interconnects, which are fixed to threesubstantially equidistant vertebrae, to align them. Each rigid part ofthe connecting member is fixed to an anchor member so that there arerespective elastically deformable connecting bodies between the pairs ofvertebrae. In this way, a single connecting member stabilizes threevertebrae, which reduces the time needed to assemble the stabilizingsystem as a whole and consequently the operating time. Also, because thethree vertebrae are interconnected by a single connecting member, theirmobility relative to each other is better controlled.

It goes without saying that providing connecting members having morethan three rigid parts connected together by elastically deformableconnecting bodies would not depart from the scope of the invention.

1. A connecting member for stabilizing the spine and maintaining thespacing between at least two anchor members, said connecting memberhaving a longitudinal axis and comprising: two integrally formed rigidrod-like parts made of a first material, wherein each of said rigidrod-like parts has a first end and a second end and comprises: afastening portion with a plurality of openings at said first end; and asolid cylindrical fixing portion at said second end and being insertableinto an anchor member, wherein said anchor member has: a threaded shankfor screwing into a vertebra; and a head that is oriented to receive theconnecting member, wherein said solid cylindrical fixing portion isinsertable into an opening of said head of said anchor member, whereinsaid rigid rod-like parts are aligned with each other and said fasteningportions face each other, and wherein said solid cylindrical fixingportions define terminal ends of the connecting member; and a connectingbody monolithically formed from a second material, wherein theconnecting body is fully disposed intermediate the two rigid rod-likeparts, wherein the second material is more elastically deformable thansaid first material, wherein said connecting body has two ends, whereinsaid two ends have protrusions that mesh with said openings of thefacing fastening portions, wherein said connecting body is able todeform elastically, and wherein the connecting body and the twointegrally formed rigid rod-like parts are aligned along thelongitudinal axis of the connecting member.
 2. The connecting member ofclaim 1, wherein the rigid rod-like parts are mechanically connectedtogether by only a single connecting body.
 3. The connecting member ofclaim 1, further comprising at least an additional connecting body andat least an additional rigid rod-like part along the longitudinal axisof said member, wherein each rigid rod-like part situated between twoconnecting bodies has one fixing portion and two fastening portions. 4.The connecting member of claim 1, wherein each of the fastening portionsof said rigid rod-like parts connected by said connecting body has afastening wall to which said connecting body is adapted to adhere andwherein said openings are formed in the fastening walls of the fasteningportions of said rigid rod-like parts.
 5. The connecting member of claim4, wherein the openings cooperate with said protrusions on saidconnecting body to increase the contact area between said fastening walland said connecting body.
 6. The connecting member of claim 5, whereinthe asperities on said connecting body are formed in situ.
 7. Theconnecting member of claim 5, wherein the openings of the fastening wallare parallel to the longitudinal axis of the connecting member.
 8. Theconnecting member of claim 1, wherein the second material of saidconnecting body is obtained by polymerization.
 9. The connecting memberaccording to claim 1, wherein the first material of which said rigidrod-like parts are made is a titanium alloy.
 10. The connecting memberaccording to claim 1, wherein the cross-section of said rigid rod-likeparts is circular.
 11. A vertebral stabilization system for fasteningtogether at least two vertebrae each having a median plane substantiallyperpendicular to the axis of the spine of which they are part and aposterior wall defining a posterior median plane of said spine, saidsystem comprising: at least two anchor members each adapted to be fixedto the posterior wall of a vertebra so that an axis which intersectssaid two anchor members is substantially parallel to the axis of thespine, the system further comprises: at least one connecting membercomprising: two integrally formed rigid rod-like parts made of a firstmaterial, wherein each of said rigid rod-like parts has a first end anda second end and comprises: a fastening portion with a plurality ofopenings at said first end; and a solid cylindrical fixing portion atsaid second end and being insertable into an anchor member, wherein saidanchor member has: a threaded shank for screwing into a vertebra; and ahead that is oriented to receive the connecting member, wherein saidsolid cylindrical fixing portion is insertable into an opening of saidhead of said anchor member, wherein said rigid rod-like parts arealigned with each other and said fastening portions face each other, andwherein said solid cylindrical fixing portions define terminal ends ofthe connecting member; and a connecting body monolithically formed froma second material, wherein the connecting body is fully disposedintermediate the two rigid rod-like parts, wherein the second materialis more elastically deformable than said first material, wherein saidconnecting body has two ends, wherein said two ends have protrusionsthat mesh with said openings of the facing fastening portions, whereinsaid connecting body is able to deform elastically, and wherein theconnecting body and the two rigid rod-like parts are aligned along thelongitudinal axis of the connecting member; wherein the two rigidrod-like parts of the connecting member interconnect said two anchormembers so that the axis of said connecting member is substantiallyparallel to the axis of the spine, wherein said vertebrae, which areinterconnected via their posterior portions, present relative mobilityalong said axis of said spine.